Electroless deposition of various metal films (including Ni, Al, Cu, Au and Pd) from liquid chemical precursors consisting of metal ions and reducing agents has been demonstrated. For example, it has been shown that selective electroless deposition of copper on surfaces of metals such as Al, Wsix, etc. is accomplished when the substrates are immersed in liquid baths containing solutions of copper EDTA+formaldehyde, etc. Insulating surfaces such as SiO.sub.2, Si.sub.3 N.sub.4, and BPSG do not get coated with copper.
As a second example, when using this technique to deposit Ni onto Al, three critical aspects must be considered. First, the Al surface must be free of oxide, which can be accomplished by performing a dilute HF dip. Second, the Al surface must be activated since Al is not catalytic for Ni deposition, which can be done by first depositing Pd. Third, the solution to deposit the Ni containing the concentrations of Ni ions and reducing agents will determine the deposition rate. Deposition is followed by an anneal step to remove traces of the solution while improving the adhesion of Ni to the Al.
These examples have common problems in that by using a conventional liquid bath arrangement, the supply of liquid chemicals is not very well controlled and the process does not meet the across the wafer uniformity and cleanliness standards required for advanced ULSI metallization. Also, low life time chemical solutions/mixtures cannot be used in a bath as, unfortunately, chemical degradation will occur.
The following two articles discuss electroless deposition and are herein incorporated by reference. One article entitled "SELECTIVE ELECTROLESS METAL DEPOSITION FOR INTEGRATED CIRCUIT FABRICATION" by Ting et al., J. Electrochem. Soc., Vol. 136, No. 2, February 1989, pp. 456-461, discusses selective electroless metal deposition process compared to selective metal deposition by chemical vapor deposition (CVD). Another article entitled "ON THE TEXTURE OF ELECTROLESS COPPER FILMS" by Junginger et al., Solid State Science And Technology, Vol. 135, No. 9, September 1988, pp. 2304-2308, discusses the textures of electroless copper films as a function of bath parameters and of the plating base.
The present invention allows for the use of chemically incompatible liquids (thus unsuitable for deposition using a liquid bath as they will react with one another once they come in contact to another) to deposit quality films on a substrate surface. The present invention also allows the use of chemically incompatible liquids for the etching of films.